At the end-of-festival press conference, in answer to a question about whether the industry was becoming more accommodating for female talent, Chastain gave an articulate and frank response about the films she had seen over the last 10 days.

“The one thing I really took away from this experience is how the world views women, from the female characters that I saw represented,” she said, a little shaky with nerves. “It was quite disturbing to me, to be honest. I do hope that when we include female storytellers we will have more of the women that I recognise in my day-to-day life. Ones that are proactive, have their own agency, don’t just react to men around them. They have their own point of view.”

Months later, in a suite at Claridge’s hotel in London, Chastain is cross-legged on the sofa, a blanket over her lap, sipping Earl Grey tea. Was she as nervous as she appeared to be when she said those things at Cannes? “Oh, yeah,” she admits, wide-eyed. “I did not expect to say it. I was very nervous as I was talking, because I was formulating my words, and it was actually coming from a very true place. And then after I spoke, every other woman on the jury spoke to the same thing. The one big regret, from whatever came out of Cannes, was that their statements did not also go around. But yes, I was not alone in what I felt being on that jury.”

In theory, we are here to talk about Molly’s Game, the high-energy directorial debut of Aaron Sorkin, which tells the true story of Molly Bloom, played by Chastain. Bloom is a former professional skier who ran high-stakes underground poker games for the ultra-rich and famous, until the FBI brought it all crashing down. While the trailer makes it look like an action romp, it has more depth than that, posing interesting questions about masculinity, wealth and power, how women can exist within that world, and the concessions they have to make to do so. It’s a Hollywood fable by any other name, and it couldn’t be more timely.

“I know,” Chastain says, wearily. “[Sorkin] said that when he was writing this film, he knew it was important. And he couldn’t articulate why.” The answer, it turns out, is that in those poker games, we see a microcosm of patriarchal society. “Maybe a month ago, a friend of mine had seen the film, and she sent me the definition of ‘patriarchy’.” She laughs. “And I was like, oh my gosh, that’s exactly what the film is.”

Chastain likes to say: “Oh my gosh.” She is giggly, at times. There is an apple-pie wholesomeness to her that adds to the impression some have formed of a movie star from another era. She is poised and immaculate, and has the kind of charismatic sunbeam dazzle of the very famous and assured. Her diction is sometimes curious, as if English is not quite her first language, though she was raised in California and now lives in New York (she has attributed it to her Italian husband). Her old-fashionedness shines through in her roles, from her star-making turns in The Help and The Tree of Life, even through to the opaque CIA agent Maya in Zero Dark Thirty, for which she earned a second Oscar nomination. For Molly’s Game, she is expected to receive a third.

Yet for all of her anachronistic qualities, Chastain is an ultra-contemporary movie star who has stepped up to the demands of what that means in 2017.

In the months that followed Cannes, she became a kind of spokesperson for women in film, a role that became more pronounced following the revelations of widespread sexual harassment and assault in Hollywood. “I can’t imagine anyone releasing a movie right now and not having everyone talk about it,” she concedes. “But this is really important for me, that this does not get swept under the rug of ‘this is a Hollywood issue’. Because this is a societal issue. Of course it’s happening in the industry, but it’s happening in the White House, it’s happening in Wall Street, it’s happening with farmworkers. By diminishing it, by saying this pertains only to this part of society, it’s dismissing all the other women who are going through harassment.” She offers to send me a letter signed by 700,000 female farmworkers across America, telling of their own experiences of harassment, standing in solidarity with the actors who have spoken out. “It’s really powerful and it’s important that the voices of women in society that are usually ignored, aren’t ignored.”

She understands that, given its notoriety, Hollywood will be a lightning rod for discussions around harassment, but she would like the focus to be wider. “I really wish that focus would also be on men. I think there’s a lot of focus on women, and I’m so happy that Time magazine’s Person of the Year was the #MeToo movement. But when you’re talking about statistics, and they say, this percentage of women are sexually harassed or raped, they don’t actually put those percentages for men – this percentage of men sexually harass. I think we need to take that focus of victimisation off of the victims and actually look at the problem. Where does it stem from?”

In October, she put that into action, when, at Elle’s Women in Hollywood event, just days after the Weinstein stories had broken, she gave an extraordinary speech in which she spoke of male co-stars who had turned up on-set intoxicated, and who had held up filming because they wouldn’t stop playing video games. “It’s unchecked power,” she says. “It’s society that allows it to continue. It’s society that doesn’t say, hold up, this is not appropriate behaviour. The sad thing is, the majority of my career has been with the most incredible men. I’ve never had a sense of being ‘less than’ because of my gender, on their film sets. And yet, the small percentage of people who use their power to victimise and abuse have such a wide reach that ... I feel bad to say, but it’s kind of put this stain on men.” Has it not also put a stain on Hollywood, on the whole industry? “I love consequences,” she smiles, diplomatically. “I mean, that’s how you grow, right?”

It took some time for Chastain to gain enough confidence to actually speak out about the issues that matter to her. “When I first started in the industry in 2011, I was afraid to speak at all. I was afraid to say anything.” But when Zero Dark Thirty came out, and she was up for a number of best actress awards, she was pitted against Jennifer Lawrence in a media-fabricated feud. “She came up to me at one event and said, ‘I hear we’re in a feud.’ And I had this moment where I was like, ‘Oh, I get it.’ This is what the media does with women. You try to divide and conquer women so you can intimidate and victimise them. This is why they don’t make movies where a lot of women get to be on set together. It’s about dividing. And at that moment I was like, well, I’m not going to be a part of that. So I immediately wrote on social media, ‘That’s a lie.’ Ever since then, I’ve just spoken up.”

She seems to have a strong sense of justice and fairness, of what’s right and wrong. “Mmm,” she says. Why is that? “I was raised by a single mom and I just saw a lot of the difficulties she faced in her life, and in what she did to try to help us have a better life than she had. Perhaps that had to have influenced me in some way.” I ask her if she has experienced any consequences of being outspoken, and being who she is. One of the many shocking revelations around the Weinstein scandal was how punitive and ruthless the industry appeared to be for those who did not stick to its murky rules. “Listen,” says Chastain, beaming her enormous smile once again. “You can be quiet and make everyone happy and that’s fine, and hopefully everyone will come see your films and you’re not going to offend anyone. But then, are you moving the needle in any direction? What’s the point of being alive, in some sense? You’re just kind of sleepwalking through your life.” She would rather be an active participant, she says. “I’m sure I have [seen consequences], but my career is doing OK.”